1. Field of the Invention
The present invention relates generally to the field of information storage devices, and more particularly to the manufacture of gimbals that brace air bearing sliders in such devices.
2. Description of the Prior Art
Information storage devices are used to retrieve and/or store data in computers and other consumer electronics devices. A magnetic hard disk drive is an example of an information storage device that includes one or more heads that can both read and write, but other information storage devices also include heads—sometimes including heads that can not write.
FIG. 1 illustrates a typical magnetic hard disk drive. The disk drive includes a head disk assembly (HDA) 10 and a printed circuit board assembly (PCBA) 11. The head disk assembly 10 includes a disk drive base 12 and a cover 14 that collectively house at least one magnetic disk 16. The disk 16 contains a plurality of magnetic tracks for storing data. The tracks are typically disposed upon opposing first and second disk surfaces 18, 20 of the disk 16 that extend between an inner disk edge 22 (at an inner diameter) and an outer disk edge 24 (at an outer diameter) of the disk 16. The head disk assembly further includes a spindle motor 26 for rotating the disk 16. The spindle motor 26 includes a spindle motor hub that is rotatably attached to the disk drive base 12. The hub has an outer hub flange that supports a lowermost one of the disks. Additional disks may be stacked and separated with annular disk spacers that are disposed about the hub.
The head disk assembly further includes a head stack assembly 28 rotatably attached to the disk drive base 12. The head stack assembly 28 includes at least one head, typically several, for reading and writing data from and to the disk 16. The printed circuit board assembly 11 includes a servo control system for generating servo control signals to position the head stack assembly 28 relative to tracks disposed upon surfaces 18, 20 of disk 16.
In a magnetic hard disk drive, the head typically comprises a body called a “slider” 42, 44 that carries a magnetic transducer on its trailing end. The magnetic transducer typically includes an inductive writer and a magnetoresistive read element. In a magnetic hard disk drive, the transducer is typically supported in very close proximity to the magnetic disk by a hydrodynamic air bearing. As the motor 26 rotates the magnetic disk 16, the hydrodynamic air bearing is formed between an air bearing surface of the slider 42, 44 and a surface 18, 20 of the magnetic disk 16.
The head stack assembly 28 includes a rotatable actuator 30. In FIG. 1, the actuator 30 includes an actuator body 32 and actuator arms 34, 36 that extend from the actuator body 32. Distally attached to the actuator arms 34, 36 are head gimbal assemblies 38, 40. The head gimbal assemblies 38, 40 respectively brace sliders 42, 44. It is contemplated that the number of actuator arms may vary depending upon the number of disks and disk surfaces utilized.
The actuator body 32 includes a bore, and the actuator 30 further includes a pivot bearing cartridge 46 engaged within the bore for facilitating the actuator body 32 to rotate about an axis of rotation 48. The actuator 30 further includes a coil support 50 that extends from one side of the actuator body 32 opposite the actuator arms 34, 36. The coil support 50 is configured to support a coil 52. A pair of magnets 54, 56 is supported by mounts 58, 60 which are attached to the disk drive base 12 (magnet 56 is indicated by the dashed lead line and it is understood that the magnet 56 is disposed underneath the mount 60). The magnets 54, 56 may be attached to the disk drive base 12 through other arrangements, such as the magnet 56 being directly mounted to the cover 12 which is mechanically engaged with the disk drive base 12. The coil 52 interacts with the magnets 54, 56 to form a voice coil motor for controllably rotating the actuator 30.
Now referring to FIG. 2, a typical HGA 38 in a magnetic hard disk drive includes a load beam 51, a gimbal 65 attached to a distal end 55 of the load beam 51, a slider 42 attached to the gimbal 65, a swage mount (not shown in FIG. 2) attached typically by spot welding to a proximate end 53 of the load beam 51, and an electrical connection of the transducer to other circuitry (e.g. a pre-amplifier) in the disk drive. The swage mount is used to attach the proximate end 53 of the load beam 51 to the actuator arm 34, typically by means of plastic deformation of a cylindrical flange that protrudes from the swage mount in alignment with hole 57. The load beam has bend areas 59 that serve a spring function that provides a preload force, also known as the “gram load,” that forces the air bearing surface of the slider 42 towards the surface 18 of the spinning disk 16. Stiffening ribs 61 can be used to practically confine the bending required to provide the “gram load” to the bend areas 59. The load beam 51 and gimbal 65 also serve a multi-degree-of-freedom hinge function that permits the slider 42 to follow the contour of the surface 18 of the spinning disk 16.
The electrical connection (not shown in FIG. 2) of the transducer to other circuitry (e.g. pre-amplifier) typically includes conductive traces laid on a dielectric layer such as a polyimide film formed on the head gimbal assembly. The dielectric layer electrically insulates the conductive traces, which may be formed of copper for example, from the gimbal, which may be formed of stainless steel for example. There are typically four or more conductive traces required for the operation of the read/write transducer. The conductive traces are electrically connected to the transducer at a trailing end of the slider. Such conductive traces are typically formed upon the dielectric layer through a deposition and/or etching process. The conductive traces include terminal pads which are disposed adjacent the slider. Various electrical connection techniques may be used to connect the terminal pads to the slider, such as gold ball bonding or wire bonding.
The slider 42 is typically adhesively bonded to the gimbal 65 using structural and conductive epoxies. The structural epoxy is used to adhere a bonding surface of the slider 42 to a tongue feature of the gimbal 65. The conductive epoxy (such as silver-doped epoxy) is typically applied to provide an electrical ground path to the slider 42 through the actuator 30, load beam 51, and gimbal 65, from the disk drive base 12.
A magnetic hard disk drive as described in the paragraphs above, however, is not the only type of information storage device that has utilized air bearing sliders. For example, air bearing sliders have also been used in the HGAs of optical information storage devices to position an objective lens over non-magnetic media for optical read-back of data. In any case, the purpose of the gimbal is to allow the air bearing slider to pivot so that its air bearing surface can closely comply with the plane of the spinning disk surface.
The thickness of the air bearing, and therefore the spacing between the transducer or objective lens and the disk surface, depends in part on the longitudinal out-of-plane curvature of the air bearing surface, commonly known as the “crown” of the air bearing surface. Since the air bearing thickness (also known as “flying height”) is a design parameter that affects the performance of an information storage device, it is desirable that the crown of the slider be tightly controlled. Consequently, when a head comprising a slider and transducer or comprising a slider and objective lens is bonded to a gimbal, creating an assembly herein referred to as an HGA, it is desirable that the assembly does not render the slider's crown to be excessively sensitive to environmental conditions such as temperature.
Typically a different material is used for the slider than is used for the gimbal to which the slider is bonded, and those different materials have different coefficients of thermal expansion. For example, often the slider is fabricated from a ceramic material such as AlTiC whereas the gimbal is fabricated from a metal such as stainless steel. Therefore, a temperature induced stress can be imparted to the slider by the gimbal when the environmental temperature changes. The temperature induced stress, in turn, can change the slider's crown and consequently alter the slider's flying height. Since information storage devices typically must operate at any temperature within a specified range of temperatures, it is undesirable for slider crown to be too sensitive to changes in temperature.
Attempts to render the slider crown less sensitive to changes in temperature have been made in the past. For example, in Japanese patent application number 07063757 published on Dec. 8, 1995, Makoto et al. suggested a combination of materials to be used for the gimbal and slider that would provide some advantageous relationship between the coefficient of thermal expansion of the gimbal material relative to the coefficient of thermal expansion of the slider material. Another example of this approach can be found in Japanese patent application number 05210228 to Kotaro et al., published on Mar. 10, 1995.
However, the coefficient of thermal expansion matching approach has an important drawback; it constrains, often impractically, the material selection for the slider, or the gimbal, or both. There are many other engineering factors that affect the selection of the best materials for the slider and for the gimbal. Often the slider material is necessarily the same material that is used as the wafer (upon which many thousands of read/write transducers are deposited by photolithographic methods). The material for such a wafer is constrained by many engineering considerations such as surface finish, electrical and magnetic properties, chemical properties, and the various mechanical properties that determine a later ability to precisely dice the wafer into many sliders with acceptable dimensional characteristics and low residual stresses. AlTiC is often a practical material choice for wafers in magnetic hard disk drive applications. The gimbal material must also have material properties that are acceptable for low cost fabrication of many small, thin, clean and non-corrosive gimbals having complex contours that maintain precise dimensions over a population of parts. Stainless steel is often a practical material choice for gimbals in magnetic hard disk drive applications. More exotic materials may not be practical where industry profit margins are traditionally low. Ultimately, there is rarely enough design freedom in the selection of materials to use a coefficient of thermal expansion matching approach to render slider crown insensitive to temperature changes.
An easier problem to solve is to compensate for (or minimize) an undesired change to slider crown caused during assembly, typically immediately after the slider is bonded to the gimbal, due to the shrinkage or expansion of a bonding adhesive as it dries or hardens. This problem is easier to solve because assembly can take place in a controlled environment characterized by known temperatures, and the shrinkage or expansion can be predicted as a function of the adhesive properties (and/or the resulting repeatable change in slider crown can be measured), and then component design or aspects of the assembly process can be changed to compensate for the repeatable change in crown.
Strategies to compensate for or minimize the crown imparted to the slider due to adhesive shrinkage or expansion during assembly have been disclosed in the art. For example, in Japanese patent application number 03297714 published on Apr. 30, 1993, Fujii Naoki disclosed a method to adjust nominal crown shape imparted to a slider when it is bonded to a gimbal. In Japanese patent application number 03070907 published on Oct. 28, 1992, Kuwamoto Yoshino disclosed a slider surface texture characterized by slits that would allow crown to be intentionally adjusted based on adhesive shrinkage. In U.S. Pat. No. 5,467,236, Hatanai and Takahashi disclosed a gimbal tongue having a non-contact portion that obstructs the flow of adhesive when the slider is bonded to the gimbal tongue, thereby confining the adhesive to a limited bonding area and preventing flow of the adhesive towards the transducer carried by the slider. Although this approach may be effective in confining the adhesive that bonds the slider to the gimbal tongue during assembly, it would have an important practical drawback if one were to try to use it as a means to reduce the ultimate temperature sensitivity of slider crown. Reducing the sensitivity of slider crown to external stresses merely by confining the bonding area is an unfavorable engineering tradeoff. Too large a bonding area simply leads to too much crown sensitivity, and too small a bonding area excessively weakens the bond's ability to resist applied forces and torques.
Therefore, what is needed is a gimbal design that reduces slider crown sensitivity to temperature changes, the sensitivity reduction being based on a novel geometry rather than material selection, and where the novel geometry does not substantially sacrifice the ability of the bond between the gimbal tongue and the slider to resist applied forces and torques.